Serissa was capable of seeming very motherly and kind, able to make Calianna feel like she belonged and was loved amongst the Cult of the Caustic Heart. However, Serissa was actually very manipulative and used her skills to convince people to sacrifice innocents to Tiamat.
"Stalker in the Swamp" (2x21) Edit
Calianna said that Serissa was motherly and kind to her, encouraging the cult to treat her as special. She was able to convince her it was the way of the world to kill people - merchants and other innocents - in the Scaled Tyrant's name. Calianna came to realise Serissa and the cult were evil and ran away from them.